The present invention relates to a supercharge control apparatus and a supercharge control method for a supercharged internal combustion engine that converges intake air fed from superchargers and supplies the intake air to cylinders.
A supercharger for improving the intake efficiency of an internal combustion engine includes an exhaust-gas driven turbocharger, which supercharges cylinders by an energy of exhaust-gas (for example, Japanese Laid-Open Patent Publication No. 2001-342840, Japanese Laid-Open Patent Publication No. H02-305322). The turbocharger includes a variable displacement turbocharger that adjusts the flow velocity of exhaust gas by selectively opening and closing a vane in a turbine. Adjusting the flow velocity of exhaust gas controls the boost pressure.
When the ratio of the pressure downstream-the compressor to the pressure upstream the compressor becomes excessively high in the turbocharger, surge occurs and intake air flows backward. According to the invention disclosed in Japanese Laid-Open Patent Publication No. 2001-342840, which uses a variable displacement turbocharger, the ratio of the pressure downstream the compressor to the pressure upstream the compressor and the flow rate of air passing through the compressor are detected. The pressure ratio is controlled in accordance with the detected information such that the pressure ratio and the air flow rate do not exceed a surging line.
The publication No. 2001-342840 discloses a supercharge control for a single super charger. The publication No. H02-305322 discloses a supercharge control when converging air fed from several superchargers and supplying the air to cylinders.
In a case where the air fed from several superchargers are converged and supplied to cylinders, if the supercharging capability of each supercharger differs from one another, the supercharger having a low supercharging capability (that is, the supercharger that supplies less amount of air to the cylinder) is likely to cause a surge. This is because even when the supercharging capability of each supercharger differs from one another, the pressure ratio of each supercharger is substantially the same. That is, as compared to the supercharger that has a high supercharging capability and a high flow rate of air supply, the boost condition represented by the combination of the pressure ratio and the air flow rate tends to easily exceed the surging line in the supercharger that has a low supercharging capability and a low flow rate of air supply although with the same pressure ratio. Therefore, in a case where intake air fed from several superchargers are converged and supplied to the cylinders, there is a limitation in increasing the pressure ratio to increase the flow rate air supply to the cylinders due to the variation of the supercharging capability of the supercharges.